This invention relates to a method and apparatus in which an electromagnetic probe is used to measure material properties in a ferromagnetic material, for example stress, or to measure the separation of the probe from the surface of such a material.
The stresses in structures such as rails, bridges and pipelines, complex mechanisms such as vehicles and machinery, or simple devices such as struts, cables or bearings arise from various causes including changes of temperature, and the loads and pressures due to use. There may also be residual stresses arising from the fabrication of the structure or device, and any bending that the structure or device was subjected to during construction; the residual stresses arising from fabrication will also be affected by any stress-relieving heat treatment. In some situations (such as pipelines) the principal stress directions can be expected to be in particular directions (circumferential and longitudinal), whereas in other situations the stress directions are also unknown. A variety of magnetic techniques are known to have some sensitivity to stress, although magnetic measurements are usually also affected by other material properties such as microstructure. A way of measuring stress in a steel plate is described in GB 2 278 450, this method using a probe containing an electromagnetic core to generate an alternating magnetic field in the plate, and then combining measurements from two sensors, one being a measure of stress-induced magnetic anisotropy, and the other being a measure of directional effective permeability (DEP). The probe is gradually turned around so the magnetic field has a plurality of different orientations in the plate, and these measurements are taken at each such orientation. The DEP signals are affected not only by stress, but also by the lift-off from the surface (i.e. the gap or separation between the probe and the surface), and so must be corrected for lift-off.
According to the present invention there is provided a method for measuring biaxial stresses in an object of ferromagnetic material using at least one probe, the or each probe comprising an electromagnet means and a magnetic sensor arranged for sensing a magnetic field due to the electromagnet means; the method comprising detecting signals from the magnetic sensor and resolving them into first and second components that are orthogonal in phase, and mapping the first and second components directly into apparent stress and liftoff components; taking such measurements with the one said probe oriented with two principal stress axes of the object, and deducing two corresponding values of apparent uniaxial stress; and hence deducing the true biaxial stresses in said object.
The mapping requires a preliminary calibration, with a specimen of the material, to determine how the first and second components of the signal vary with lift-off (at a constant stress) and vary with stress (at a constant lift-off), and deducing from the calibration measurements the applicable mapping for any stress and any lift-off. The signals from the sensor are at the frequency of the alternating field, and the components may be the components in phase with the current supplied to the electromagnet means, and the component in quadrature to that. The mapping may be represented in the impedance plane (i.e. on a graph of quadrature component against in-phase component) as two sets of contours representing signal variation with lift-off (for different values of stress) and signal variation with stress (for different values of lift-off), the contours of both sets being curved. The contours or lines of one set intersect those of the other set at non-orthogonal angles. The angles at which the contours for constant lift-off (varying stress) intersect any one contour for constant stress (varying lift-off) are constant along that contour. However the angles of intersection of different lift-off lines with any one stress line are different: the angle of intersection at a fixed lift-off varies slightly with stress. Hence measurements taken along a few contours of each set enable the positions of the other contours of each set to be determined.
Surprisingly this simple mapping has been found to give an accurate representation of the variation of the signals with material property (e.g. stress or microstructure); more surprisingly it enables these variations to be distinguished unambiguously from variations arising from lift-off or other geometrical variations such as surface texture or curvature. It may perhaps be presumed that material property changes cause changes in the permeability of the steel in the magnetic circuit and these have both inductive and resistive (lossy) components, whereas geometrical changes such as lift-off change the amount of air in the magnetic circuit, in which energy dissipation by eddy currents cannot occur, so this of itself would be purely inductive (non-lossy). Where the object has a coating of a non ferromagnetic material (such as a paint, or a ceramic), this coating will separate the probe from the surface, so the lift-off measurement may indicate its thickness.
Preferably the electromagnet means comprises an electromagnetic core and two spaced apart electromagnetic poles, and the magnetic sensor is preferably arranged to sense the reluctance of that part of the magnetic circuit between the poles of the electromagnet means. It is also desirable to arrange for such measurements to be taken with a plurality of different orientations of the magnetic field, at a single location on the object. This may be achieved using a single probe that is rotated at that location, measurements being taken with different orientations of the probe, or using an array of probes of different orientations that are successively moved to that location. In either case, the sensor or sensors provide a measure of the permeability of the material through which the flux passes between the poles, and so provide a signal indicative of the effective permeability of the material; the corresponding measurements at different probe orientations at a location on the object hence indicate the effective permeability in different directions. The signals from this sensor may be referred to as a reluctance, or ‘flux linkage’, signal.
The probe, or at least some of the probes, may also include a second magnetic sensor between the two poles and arranged to sense magnetic flux density perpendicular to the direction of the free space magnetic field between the poles. This second sensor would detect no signal if the material were exactly isotropic; however stress induces anisotropy into the magnetic properties of the material, and so the signals received by the second sensor are a measure of this stress-induced magnetic anisotropy or ‘flux rotation’. The variations in the flux rotation signals at different probe orientations, at a location on the object, enable the directions of the principal stress axes to be accurately determined. The flux rotation signals can also be related to the stress.
The flux linkage signal from the or each probe is preferably backed-off, i.e. processed by first subtracting a signal equal to the signal from that sensor with the probe adjacent to a stress-free location. The backed-off signal is then amplified so the small changes in the flux linkage signal due to stress are easier to detect. This backing off is performed after resolving into in-phase and quadrature components but before performing the mapping. Preferably the signals from the or each probe are digitized initially, and the backing-off and resolution are performed by analysis of the digital signals.
To achieve penetration below the surface of the ferromagnetic object it is desirable to operate at alternating frequencies less than 200 Hz, for example between 5 Hz and 100 Hz (which in mild steel provide penetrations of about 5 mm and 1 mm respectively). In other situations, where such depth of penetration is not required, higher frequencies can be used, for example up to 150 kHz for a penetration of only about 15 μm. The depth of penetration may be represented by the skin depth, δ=1/√{square root over (()}πμoμrfk), where μo is the permeability of free space, μr is the relative permeability of the material, k is its electrical conductivity, and f is the frequency. The frequency should be such that the skin depth is much less than the thickness of the object.
Generally, the more different probe orientations are used for taking measurements the more accurate the determination of stress levels and principal axes can be. In many cases the principal stress axes can be assumed to be aligned in particular directions—axial and circumferential directions in the case of a pipe, for example—so that the signal maxima for flux linkage signals would be expected to be along these directions, and the signal maximum for flux rotation signals would be along the bisection angles between these directions.
The probe, or at least some of the probes, may also include a third magnetic sensor (a ‘flux leakage’ sensor) between the poles and arranged to sense magnetic flux density parallel to the free space magnetic field. This third sensor will detect any flux leakage, this being influenced by changes in material properties, lift-off, and cracks. As with the flux-linkage sensor, measurements at a location are preferably made at different probe orientations.